Sebastian Rost, Greg Houseman, Andrew Frederiksen, David Cornwell , Metin Kahraman, Sleda Altuncy Poyraz, Ugur Teoman, David A. Thompson, Niyazi Türkelli, Levent Gülen

Information on fault zone structure is essential for our understanding of earthquake mechanics, continental deformation and our understanding of seismic hazard. We use the scattered seismic wavefield to study the subsurface structure of the North-Anatolian Fault Zone (NAFZ) in the region of the 1999 {\.I}zmit and D\"uzce rupture using data from an 18-month dense deployment of seismometers with a nominal station spacing of 7~km. Using the forward and backscattered energy following the direct \textit{P}-wave arrival from teleseismic earthquakes, we apply a scattered wave inversion approach and are able to resolve fine-scale changes in lithospheric structure on scales of 10~km or less in an area of about 130~km by 100~km across the NAFZ. We find several crustal interfaces that are laterally incoherent beneath the surface strands of the NAFZ and evidence for contrasting crustal structures either side of the NAFZ, consistent with the presence of juxtaposed crustal blocks and ancient suture zones. Although the two strands of the NAFZ in the study region strike roughly east-west, we detect strong variations in structure both north-south, across boundaries of the major blocks, and east-west, parallel to the strike of the NAFZ. The shallow NAFZ is coincident with features detected on the crust-mantle boundary (Moho) and deeper into the mantle. We show that a dense passive network of seismometers is able to capture information from the scattered seismic wavefield and using a tomographic approach, resolving the fine scale structure of crust and lithospheric mantle even in geologically complex regions. Our results show that major shear zones exist as narrow structures beneath the NAFZ throughout the crust and into the lithospheric mantle, suggesting a strong coupling of strain across these depths.